This invention relates to power supply circuits for microwave ovens, and more particularly to a power supply circuit adapted to operate successfully on either 50Hz or 60Hz input power.
The use of microwave frequency energy for the heating and cooking of foodstuffs is well known, and microwave ovens are in increasingly common use in many parts of the world. Unfortunately, the frequency and voltage of the electrical power supplied to buildings has not been standardized throughout the world with the result that electrical appliances manufactured in one country may no be operable in other countries because of differing electrical power supplied. In some countries, such as Japan, the frequency of electric power varies within the country, such power being supplied at a frequency of 50Hz in some areas and 60Hz in other areas.
While the use of differing power frequencies creates problems with many electrical appliances, the problem is particularly troublesome in microwave ovens. Most microwave ovens operate at a fixed power output from the microwave generating source, such as a magnetron. The heating or cooking of a variety of foods is accomplished by varying the length of time in which the food is exposed to microwave energy. Thus, virtually all microwave ovens are equipped with a timing device and the user is instructed to heat or cook a particular food for a prescribed period of time, which time is a function of the output power level of the magnetron. Thus, the amount of time required to cook a given portion of food to a desired degree will be longer at a microwave power level of 600 watts than at 1000 watts. It is therefore important that the power level not be affected by a change in electrical input power.